1. Technical Field
The present invention relates to an image processing apparatus, an image forming apparatus, and an image processing method for generating an image signal which represents gradation, by use of an area coverage modulation method.
2. Related Art
In an electrophotographic or inkjet image forming apparatus such as a copier, a printer, or the like, input image information is subjected to screen processing by means of which pseudo-intermediate gradation is represented by a binary value showing presence/absence of an imaging dot, and a result of screen processing is output by an output device.
Documents created by such an image forming apparatus include various objects from the viewpoint of image quality; e.g., images such as photographs, graphics such as gradations or patches, characters, lines, and the like.
A parameter typifying screen processing includes the number of lines (the number of identical patterns included per inch). The number of lines optimal for the previously-described image object varies from one image object to another.
For instance, 200 lines or more are preferable for an image from the viewpoint of visual characteristics of the human eye. However, when the number of lines is large, a gradation characteristic or granularity is deteriorated by response of a marking engine. In contrast, when the number of lines is small, a screen structure becomes visible, or texture is lost. Accordingly, 170 lines to 230 lines are often used.
In relation to graphics, further emphasis is placed on a gradation characteristic or granularity, and hence 150 lines to 200 lines or thereabouts are often used. In relation to characters and lines, which are desired to include no jagged edges or discontinuities, 300 lines or more are often used.
Specifically, the maximum number of lines is desirable to smoothly represent characters, lines, and the like, but an excessively large number of lines results in deterioration of a gradation characteristic or granularity.